Pharmokinetics in practice Flashcards
what are first order kinetics?
-Rate of elimination is proportional to the plasma drug concentration
-A constant % of the plasma drug is eliminated over a unit of time
-Often, the decline is exponential –
A CONSTANT FRACTION of the drug is eliminated per unit of time
what are zero order kinetics?
- the rate of elimination is not proportional to the plasma drug concentration- the metabolism processes becomes saturated - here a CONSTANT AMOUNT of the plasma drug is eliminated over time
E.G If an enzyme system that removes a drug is saturated the rate of removal of the drug is constant and unaffected by an increase in concentration
Ethanol follows zero order kinetics once alcohol dehydrogenase has been saturated
what is half life ( t1/2) dependant on?
- it is dependant on clearance of drug fr the body by all eliminating organs ( hepatic, renal, faeces, breath)
- it is also dependant on the volume of distribution (VD)- a drug will large VD will be cleared more slowly than a drug with a small vd
what is half life ( t1/2)?
- it is the units of the rate constant are hard to use practically
- therefore we usually use half life which is the time taken for a concentration to reduce by one half
what is the value of T1/2?
0.693k=ln2
why is t 1/2 important in clinical practise?
- Drug dosing (short t1/2 will need more frequent dosing)
2.Organ dysfunction (t1/2 may be increased)
3.Adverse drug reactions or management of toxicity (how long will drug take to be removed and symptoms to resolve)
4.Short t1/2 increases risk of discontinuation/withdrawal symptoms (such drugs may need dose weaning on cessation)
when are single or STAT doses useful?
useful in treating acute condition
- effects wear off after a few minutes-hour
how are most single dose or stat doses repeated?
continuous IV infusion
repeat iv dosing
define genomics
‘The study of the genomes of individuals and organisms that examines both the coding and non-coding regions. The study of genomics in humans focuses on areas of the genome associated with health and disease.’
define pharmacogenomics
‘The use of genetic and genomic information to tailor pharmaceutical treatment to an individual.’
what is bioavailability?
the fraction of the administered drug that reaches the systemic circulation unaltered (F)
what is the bioavailability of IV drugs?
F=1
As 100% of the drug reaches circulation
what is the bioavailability of oral drugs?
F<1
if they are incompletely absorbed or undergo first pass metabolism
how is bioavailability determined?
determined by measuring plasma concentration after oral and iv doses
what is distribution?
the rate and extent of movement of a drug into tissues from blood
